A fuel cell is constituted by putting a solid polymer electrolyte film between an anode electrode and a cathode electrode to form a unit cell and stacking a plurality of unit cells by way of an electrode referred to as a separator (or bipolar plate).
For the material constituting the separator for use in the fuel cell, a low contact resistance and a property that the low contact resistance is maintained for a long time during use as the separator. For this purpose, application of metal materials such as aluminum alloys, stainless steels, nickel alloys, and titanium alloys has been investigated so far in view of fabricability and strength.
Further, it has been said that the inside of a fuel cell is an acidic corrosive environment at pH of 2 to 4, and corrosion resistance (acid resistance) of maintaining the low contact resistance even in the corrosion environment is also required for the material used for the separator.
Since metals such as stainless steel or titanium show good corrosion resistance due to their passivation films on the surface, they have been investigated as materials for the separator of the fuel cell. However, since the passivation film has a high electric resistance, when a metal such as stainless steel or titanium is used for the separator of the fuel cell, electroconductivity may possibly be degraded remarkably by the passivation film formed on the surface thereof in the corrosion environment. Accordingly, even if the contact resistance at the initial stage of use is low, the low contact resistance can not be maintained for a long time during use as the separator and the contact resistance increases with time, which results in loss of current. Further, it is also a problem, for example, that the electrolyte film is deteriorated by metal ions that dissolve from the corroded material.
In view of such problems, in order to suppress increase in the contact resistance and maintaining the electroconductivity for a long time, various kinds of metal separators have been proposed so far. For example, those prepared by gold plating on the surface of a metal separator such as stainless steel or titanium (Patent Document 1), those prepared by removing oxide films on the surface of a substrate comprising, for example, stainless steel and titanium and then depositing a noble metal or a noble metal alloy (Patent Document 2), or those prepared by coating the surface of a stainless steel substrate with a corrosion resistant metal film and coating a noble metal film thereon (Patent Document 3) have been proposed.    [Patent Document 1] JP-A No. 10-228914    [Patent Document 2] JP-A No. 2001-6713    [Patent Document 3] JP-A No. 2001-93538